[PATCH v2] x86/entry/32: Fix setup of CS high bits

[Jan Kiszka]

Even if we are not on an entry stack, we have to initialize the CS high
bits because we are unconditionally evaluating them
PARANOID_EXIT_TO_KERNEL_MODE. Failing to do so broke the boot on Galileo
Gen2 and IOT2000 boards.

Fixes: b92a165df17e ("x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack")
Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
Acked-by: Joerg Roedel <jroedel@xxxxxxx>
Reviewed-by: Joerg Roedel <jroedel@xxxxxxx>
---

Changes in v2:
 - adjust comment according to Andy's feedback
 - added JÃrg's ack/review (assuming the comment change does not affect it)

 arch/x86/entry/entry_32.S | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 2767c625a52c..fbbf1ba57ec6 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -389,6 +389,13 @@
 	 * that register for the time this macro runs
 	 */
 
+	/*
+	 * The high bits of the CS dword (__csh) are used for
+	 * CS_FROM_ENTRY_STACK and CS_FROM_USER_CR3. Clear them in case
+	 * hardware didn't do this for us.
+	 */
+	andl	$(0x0000ffff), PT_CS(%esp)
+
 	/* Are we on the entry stack? Bail out if not! */
 	movl	PER_CPU_VAR(cpu_entry_area), %ecx
 	addl	$CPU_ENTRY_AREA_entry_stack + SIZEOF_entry_stack, %ecx
@@ -407,12 +414,6 @@
 	/* Load top of task-stack into %edi */
 	movl	TSS_entry2task_stack(%edi), %edi
 
-	/*
-	 * Clear unused upper bits of the dword containing the word-sized CS
-	 * slot in pt_regs in case hardware didn't clear it for us.
-	 */
-	andl	$(0x0000ffff), PT_CS(%esp)
-
 	/* Special case - entry from kernel mode via entry stack */
 #ifdef CONFIG_VM86
 	movl	PT_EFLAGS(%esp), %ecx		# mix EFLAGS and CS
-- 
2.16.4